He's not really, he's just drunkenly WUMming
To be clear - setting aside the horrific loss of life even attempting this would incur, there is no way the United States can guarantee that it would be able eliminate all of North Korea's nuclear weapons in a preliminary strike, much less its thousands of pieces of almost equally deadly conventional artillery. Nuclear weapons are kept underground, or on moving delivery systems, for precisely this reason. You might get lucky and take out one or two, but not all. They have not invested decades into developing them without it having occurred to them how to keep their deterrent safe. Even the Trump (!) military command realises this, and they've more or less stated so publicly on many occasions.
Anyone who states otherwise is bullsh+tting and fantasizing - lest someone reading the above exchange confuse for this a reality-based discussion.
I still don't really think anything will actually happen... but in a way this is far more dangerous than the Cuban Missile Crisis was, though still much further from the brink, simply because we know so much less about North Korea. But, needless to say, while everyone sentient on the planet has long since realized that the 'Leader of the Free World' is a lazy, ignorant, childish, oaf, his twitter flailing runs the risk of being legitimately misinterpreted in Pyongyang.
And he is completely out of control. His own generals were sincerely worried that what became the transgender "ban" (ie legally meaningless tweet)
was actually going to have been him declaring war on someone via the internet.
